For switching of high load currents switch devices, for example based on metal oxide semiconductor (MOS) transistors, are used in some applications. For many applications, it is desirable that a resistance of the switch device in a switched-on state (i.e. a state where the switch device is conducting) is as low as possible to reduce power dissipation in the switch device.
In some applications, it is required to be able to measure the load current flowing via the switch device. For some applications, a high dynamic range from low load currents to high load currents may be required. To determine the load current, in some applications a voltage drop across the switch device is measured. For example, in case of a MOS transistor being used as a switch device, a drain-source voltage may be directly or indirectly measured to obtain a measure for the load current. However, in case of a low-ohmic switch device, i.e. a switch device having a low resistance value when switched on, for low load currents the voltage drop across the switch device becomes very small, which may lead to inaccuracies in the measurement.
In some conventional approaches using a switch transistor like a MOS transistor or switch device, for measuring a load current via the switch transistor a measurement transistor which is scaled with respect to the switch transistor may be used. In some approaches, a drain-source voltage of this further transistor is regulated to correspond to the drain-source voltage of the switch transistor. In such a case, a current through the measurement transistor is scaled with respect to the load current corresponding to the scaling factor between the transistors. However, even in such a circuit the regulating of the drain-source voltage of the measurement transistor to the drain-source voltage of the switch transistor has inaccuracies, which may become more pronounced for small load currents in case the switch transistor has a low resistance. On the other hand, as mentioned above, switch devices having a low resistance are generally desirable to reduce power dissipation.